FIG. 1 shows a nuclear steam generator 52 which comprises a plurality of U-shaped tubes disposed within its shell to form tube bundles 50. Tube bundles 50 typically contain thousands of generator tubes. Tube sheets are provided for supporting the tubes at the ends opposite the U-shaped curvature. A primary fluid is heated in the reactor core through circulation and enters steam generator 52 through a primary inlet 54 fluid nozzle where it is distributed by upper tube sheet through the generator tubes and collected at the lower tube sheet. The fluid then flows through a primary fluid outlet 56 nozzle to the remainder of the reactor system. A feedwater stream is also introduced into steam generator 52 secondary side through a feedwater nozzle. The feedwater mixes, in steam generator 52, with water returning from moisture separators located above the tubes.
Tube bundles 50 also have a number of parallel tube support plates (“TSP”) 58 that are arranged in tandem and spaced along the longitudinal length of bundles 50 through which heat exchange tubes pass and are supported against vibration. These TSP 58 can take many forms. The secondary water and generated steam flow through broached openings in TSP 58. Deposition forms from fine particles of magnetite at a relatively high temperature within the secondary water and also from ionic species which collect and concentrate when the secondary water boils and leaves as steam. Significant amounts of deposition are introduced in the feedwater. The deposition may be a combination of species depending on the chemistry of the feedwater. This deposition collects and builds up as sludge patches about broached openings of TSP 58, particularly on the free tube heat transfer surface which reduces the efficiency and the blockage within the support flow holes which acts as an impediment to the flow of feedwater and the water/steam mixture in the upper parts of the tube bundle as the water converts to steam.
The flow of boiling products, liquid or steam, through a TSP 58 intersections can be restricted by the presence of deposition on the open tube surface immediately adjacent to the lower and upper edges of TSP 58, deposition within the center of the TSP broached volume, or deposition growth from the TSP broached edges to the tube surface at the lower and upper openings to the TSP flow path. This later deposition growth condition at the lower edge of the support has been termed “webbing” since it resembles a densely woven spider web formation that has a thickness dimension considerably less than the surface area dimension. While “webbing” has been observed in a very small quantity of steam generators at the lower edge of the upper most TSP for a very small percentage of tube to tube support plate intersections, also called TSP flow region, this “webbing” formation does not represent the geometry of blockage for the remaining, larger population of TSP flow regions within the steam generator. FIG. 2 presents the configuration for a trefoil broached TSP design. The measurement and analysis methods for the invention are applicable to any broached TSP design.
Deposit formation produces an alteration of the eddy current response to the TSP, however the specific changes to the signal response have not been formally measured to determine the degree of flow restriction through the available volume of the TSP flow region.
A method to determine the effect of a measured pattern (collection or population) of flow restriction for a single or set of TSP on critical steam generator operational parameters has not been formulated. A current practice is to perform extensive visual examination of the TSP flow regions. This current practice requires unacceptable amounts of time and exposes personnel to large amounts of accumulated radioactive dose. If a steam generator does not provide access ports for visual inspection tooling between each pair of TSP, the visual examination is limited to accessible areas for the uppermost TSP. Even with full access to each TSP, only a small percentage of the total available TSP flow regions can be examined with the current visual inspection tooling technology. Consequently, a complete examination of the population of TSP flow regions is not possible and subsequent analysis of the effect of observed flow restriction of steam generator operational parameters would be subject to large uncertainties.
A proposed application of a flux leakage eddy current method presupposes the existence of a specific deposition formation “webbing” as the primary and substantial cause of TSP flow region clogging within the steam generator. While “webbing” has been observed in a small quantity of steam generators, in a small quantity of TSP flow regions at the uppermost TSP, “webbing” is not the primary deposition geometry for TSP flow regions in tube support plates below the uppermost TSP. EU Patent Application PCT/FR 2009/050232 for TSP clogging proposes the use of a bobbin coil method in addition to the flux leakage method to characterize the clogging within the TSP flow regions. This application does not provide a process to relate the collective measured results of the collective TSP flow regions to any of the critical operational parameters for the steam generator. While the proposed method may yield a relative pattern of clogging for a single or set of tube support plates, the application does not address any specific use of these results to ascertain the operational characteristics of the steam generator.
U.S. Pat. No. 7,405,558, incorporated by reference herein, teaches a method for the measurement of deposition on the open tube surfaces between pairs of TSP.